Asolution of hydrochloric acid is placed on a balance. magnesium metal is added and the mass is recorded. the solution is stirred and the mass of the solution is recorded every 10 minutes. the following data is collected:
time (mins) mass of hydrochloric acid and magnesium (g)
0 160.0 g
10 156.5 g
20 152.0 g
30 147.5 g
40 142.5 g
what does the data show is occurring?
the hydrochloric acid and magnesium metal react to form a soluble compound.
the hydrochloric acid and magnesium metal do not react.
the hydrochloric acid and magnesium metal react to form a precipitate.
the hydrochloric acid and magnesium metal react to form a gas.

Remember conservation of mass.  Mass can't just suddenly show up or disappear.  It has to come from somewhere, and it has to go somewhere.

The mass of this mixture/solution is decreasing as time goes on. That
means that some mass is going somewhere, out of the dish and away
off the scale.

-- React to form a soluble compound ?
       Maybe, but that doesn't take mass away

-- They don't react ?
        Maybe, but that doesn't take mass away

-- React to form a precipitate ?
        Maybe, so big deal.  It sinks to the bottom. It doesn't take mass away.

-- React to form a gas ?
         If that happens, then the gas carries mass away !
        That must be it !